MoCA: A Monte Carlo code for Comptonisation in Astrophysics. I. Description of the code and first results

TL;DR

MoCA is a novel Monte Carlo simulation tool for Comptonisation in astrophysics, incorporating full special relativity, Klein-Nishina effects, and polarization, providing detailed spectra and polarization predictions for different corona geometries.

Contribution

This paper introduces MoCA, the first Monte Carlo code using a single photon approach with full relativistic physics, Klein-Nishina effects, and polarization in astrophysical Comptonisation modeling.

Findings

Spectra agree with analytical codes at low/moderate optical depths.

Significant differences at high optical depths due to multiple scatterings.

Polarization properties can distinguish between corona geometries.

Abstract

We present a new Monte Carlo code for Comptonisation in Astrophysics (MoCA). To our knowledge MoCA is the first code that uses a single photon approach in a full special relativity scenario, and including also Klein-Nishina effects as well as polarisation. In this paper we describe in detail how the code works, and show first results from the case of extended coronae in accreting sources Comptonising the accretion disc thermal emission. We explored both a slab and a spherical geometry, to make comparison with public analytical codes more easy. Our spectra are in good agreement with those from analytical codes for low/moderate optical depths, but differ significantly, as expected, for optical depths larger than a few. Klein-Nishina effects become relevant above 100 keV depending on the optical thickness and thermal energy of the corona. We also calculated the polarisation properties for the two geometries, which show that X-ray polarimetry is a very useful tool to discriminate between them.